Composite of pleated and nonwoven webs, method and apparatus for the electrostatic charging of same

ABSTRACT

A web or film is electrostatically charged by sequentially subjecting the web or film to a series of electric fields such that adjacent electric fields have substantially opposite polarities. Both a method and apparatus are disclosed for charging the web or film.

RELATED CASES

This application is a divisional application of U.S. application Ser.No. 08/426,031, filed Apr. 21, 1996, now U.S. Pat. No. 5,955,174, issuedSep. 21, 1999, which is a continuation in part of U.S. application Ser.No. 08/411,486, filed Mar. 28, 1995, now U.S. Pat. No. 5,686,050, issuedNov. 11, 1997, which is a continuation-in-part of application Ser. No.07/958,958, filed Oct. 9, 1992, now U.S. Pat. No. 5,401,446, issued Mar.28, 1995. The application and patents are incorporated herein in theirentirety by reference.

FIELD OF THE INVENTION

The present invention relates to novel composite webs of pleated andunpleated carded webs with nonwoven webs such as meltblown webs ofsynthetic thermoplastic fibers. The invention also relates to suchcomposites which are cold electrostatically charged and to the processof so charging.

BACKGROUND OF THE INVENTION

Electrically charged fibrous materials to be used as a filtration mediumhave been known for some time. The prior art and its shortcomings arereviewed in the patent identified above.

The currently available methods for cold charging a web also haveproblems developing the desired charge densities and, in addition,suffer from the added problem of having the charge bleed off the webwith time.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide novel compositestructures which are pleated or nonpleated carded webs co-formed into aporous composite with nonwoven webs.

It is a further object of the present invention to provide a method andapparatus for electrostatically charging such composite structures.

These and other objects which will become apparent to one skilled inthis art are achieved by the discovery of novel composite web structurescomprising pleated or unpleated carded webs with nonwoven webs.

The carded webs and the nonwoven components of the composites of theinvention can be held together as laminated structures by various meanssuch as heat fusion (e.g., at discrete points) of fibers having a lowermelting point than the melting point of the remaining webs.Alternatively, one or more webs can be provided at appropriate selectedpositions between adjacent layers of the composite structure, which uponthe application of heat, will likewise provide a satisfactory adhesivebond. The composites of the invention are of various configurations, andare particularly useful as filtration devices of various media includinggases and liquids. When electrostatically treated, these filtrationproperties are further and remarkably increased.

Webs that are produced by the methods and apparatus of the presentinvention have relatively high charge densities and if the compositescontain conductive films or fibers, they do not exhibit attendantsurface static electrical charge which would be inappropriate for use incombustible environments.

For the purposes of the discussion in connection with the electrostaticcharging which is to follow and for use in the claims, the terms"positive" and "negative" are meant to be relative terms. For example, apair of electrodes will have a positive electrode and a negativeelectrode any time there is a difference in potential between the twoelectrodes. The positive electrode, for the purposes of this discussion,will be the electrode with the more positive (or less negative)potential, while the negative electrode will be the electrode with themore negative (or less positive) potential.

Also, it is well known to practitioners in the art that the techniquesfor charging webs may be effectively used to charge films and viceversa. Therefore, for purposes of the discussion which is to follow andfor use in the claims, the terms "web" and "film" which preferably forthe principal intended uses are permeable or porous to the medium to befiltered. These terms will be considered to be interchangeable and willbe generically referred to by the term "web".

The present invention provides a method for charging a composite by themethods and the apparatus described in the aforemost mentioned patentand patent application.

The present invention is suitable for charging the multilayer compositeof the invention made of generally nonconductive polymeric materials.The composites of the nonwoven component and the carded web may be madefrom many of the thermoplastics capable of being meltblown (orspunbond). The thermoplastic resins useable in the production ofnonwovens are known and include polyolefins (elastomeric ornonelastomeric) such as polyethylene, polypropylene including highdensity polyethylene, ethylene copolymers (including EVA and EMAcopolymers with high tensile moduli), nylon, polyamides, polyesters,polystyrene, poly-4-methylpentene-1, polymethylmethacrylate,polytrifluorochlorethylene, polyurethanes, polycarbonates, silicones,polyphenylene sulfide. The fibers constituting the composites aregenerally hydrophobic, but may be a mixture of hydrophobic andhydrophilic polymeric (synthetic) materials which, overall, havehydrophobic characteristics. The mixture may include layers of differenthydrophobic and hydrophilic properties, or the fibers of such differentproperties may be commingled or intertwined (i.e., mixed). A favoredclass of polymers includes polypropylene (PP), recycled and virginpolyethylene terephthalate (PET), all types of polyethylene (PE), suchas linear low density polyethylene (LLDPE), polybutylene terephthalate(PBT), polycarbonates, polychlorotrifluoroethylene (PCTFE), andpolycyclohexyldimethylene terephthalate (PCT).

In accordance with the present invention, the components may containboth conductive and nonconductive fibers such asmeltblown/cotton/meltblown thermally bonded webs or meltblown/cottonhydroentangled or needle-punched webs, or hydroentangled mixtures ofcarded polyester staple fibers and wood tissue, such as SONTARA webs (DuPont), as well as deep-grooved polyester fibers such as 4DG fibers(Eastman Chemical Company).

The composite webs (and the laminates) of the invention may contain oneor more layers of cellulosic fibers like cotton, ramie, hemp, jute,flax, kenaf, bagasse, and eucalyptus, including derivatives of cellulosesuch as rayon, or many others, as well as "Tencel", a comparatively newfiber of Courtaulds P.C. (made from wood pulp). The composite mayinclude staple fibers (i.e., natural fiber (as of raw wool, cotton,flax, or hemp) or synthetic fibers (as cut from continuous filaments ofrayon or nylon) of relatively short length that when spun and twistedforms a yarn as distinguished from a filament).

In a preferred embodiment of the invention the composite web iscomprised of two layers including a first layer formed of a carded weband folded to define a series of pleats, and a second layer formed of anonwoven web and including a planar surface, wherein the pleats of thefirst layer are attached to the planar surface of the nonwoven web.Preferably, the carded web is comprised of a blend of fibers includingpolypropylene, polyethylene, or polyester fibers. Preferably also, thepolyolefins are meltblown like of polypropylene and the nonwoven web canbe a needlepunched nonwoven web. The first and second layer or when morethan two layers are used are heat bonded by a fiber which has a fusionpoint below that of the other fibers in the composite or by a web ofsuch fibers which performs as an adhesive web. When a compositecomprises a third layer which is generally planar, the pleats of thefirst layer are attached to the planar surface of the nonwoven web, asby heat bonding. In another preferred configuration, the third layer isformed of a carded web and folded to form a series of pleats, whereinthe pleats of the third layer are attached to a surface of the nonwovenweb opposing the planar surface which receives the first layer. Thisstructure may include a scrim layer which has a generally planarsurface, wherein the pleats of the first layer are attached as by heatbonding to the planar surface of the scrim layer.

In a more preferred embodiment of the invention, all of these, as wellas other configurations of the composites (and different compositions orconstituents therefor) are electrostatically charged composites. Thefiltration properties of these charged composites are remarkablyimproved.

BRIEF DESCRIPTION OF THE DRAWINGS

For further detail regarding the improvements of the present invention,reference is made to the following detailed description of exemplaryembodiments, in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic view of one embodiment of the present inventionfor cold charging a web;

FIG. 2 is a schematic view showing the electrical fields of a method forcold charging a web according to the prior art;

FIG. 3 is a schematic view showing the electrical fields of a method forcold charging a web according to the apparatus shown in FIG. 1;

FIG. 4 is a schematic view of another embodiment of the presentinvention for cold charging a web;

FIG. 5 is a schematic view showing the electrical fields of a method forcold charging a web according to the apparatus shown in FIG. 4;

FIG. 6 is a schematic view of an embodiment of the present invention forcold charging a web which permits varied charging of the web usingdifferent processes, both alone and in combination;

FIG. 7a is an elevational view of a unitary roller for supporting atransported web;

FIG. 7b is an elevational view of an alternative embodiment roller forsupporting a transported web;

FIG. 7c is an elevational view of a further alternative embodimentroller for supporting a transported web;

FIG. 8a is a schematic sectional view of an apparatus similar to that ofFIG. 6, with split collector shells hinged for access to the web;

FIG. 8b is a schematic sectional view of an apparatus similar to FIG. 6,with split collector shells joined to slide relative to one another foraccess to the web;

FIG. 9 is a schematic sectional view of an apparatus similar to FIG. 6,with the charging wires mounted to a fixture which can be pivoted foraccess to the web;

FIGS. 10A to 10E are schematic drawings showing pleated carded webs ofvaried construction which are suitable for electrostatic charging inaccordance with the invention;

FIG. 11 is a schematic drawing showing an apparatus for producing thepleated carded webs of FIGS. 10A to 10E; and

FIGS. 11A and 11B are schematic drawings showing alternative embodimentlappers for use with the apparatus of FIG. 11.

DETAILED DESCRIPTION OF THE INVENTION

In the description which follows the term "web" has been used forconsistency and convenience, in place of terms such as "composite web"or "folded composite" or other terminology conventionally used in thisart.

Referring now to the drawings in which like reference charactersdesignate like or corresponding parts throughout the several views, FIG.1 shows an embodiment of an apparatus 10 for cold charging a web inaccordance with the present invention. The depicted apparatus 10generally comprises an uncharged moving web 12 having a first side 14and a second side 16. The web 12 may have a triboelectric chargeassociated with the relative inherent electronegative/electropositivenature of the polymer. In addition, the web 12 may be a web that hasbeen precharged by either a hot or cold charging process. The web 12passes into the apparatus 10 with the first side 14 in contact withpositioning roller 18. The second side 16 of the web 12 then comes incontact with the first charging drum 20 which rotates with the web 12and brings the web 12 into a position between the first charging drum 20and a first charging bar 22.

In the following discussion, the first charging drum 20 is assumed to be"negatively charged" (i.e., at a negative potential) and the firstcharging bar 22 is assumed to be "positively charged" (i.e., at apositive potential). These assumptions are for the ease of describingthe operation of the invention only and are not intended to limit thescope of the invention to those specific relative potentials.

As the web 12 passes between the charging bar 22 and the charging drum20, an electrostatic charge is developed adjacent the two sides 14 and16 of the web 12. A relative negative charge is developed adjacent thefirst side 14 of the web 12 and a relative positive charge is developedadjacent the second side 16 of the web 12. The web 12 then passes on toa "negatively charged" (i.e., at a negative potential) second chargingdrum 24 and, further, to a position between the charging drum 24 and asecond "positively charged" (i.e., at a positive potential) charging bar26 where the web 12 has the polarity of its charge reversed. This effectis seen in polyolefin fibers but is not always seen in webs preparedfrom fibers of, for example, polyesters or PCTFE. That is, a relativepositive charge is developed adjacent the first side 14 of the web 12and a relative negative charge is developed adjacent the second side 16of the web 12. The charged web 12 then passes on to a positioning roller28 and out of the apparatus.

For the charging of webs, both of the charging drums 20 and 24 are heldat a relative negative charge (or grounded) while the charging bars 22and 26 are held at a relative positive charge. However, since differentsides of the web 12 are in contact with the charging drums (the secondside 16 is in contact with the first charging drum 20 and the first side14 is in contact with the second charging drum 24) the charges adjacentto the sides 14 and 16 of the web 12 are changed during the charging ofthe web 12 in the apparatus 10. Thus, the first side 14 is firstnegatively charged and then positively charged, and the second side isfirst positively charged and then negatively charged.

Without being bound by theory, it is believed that the switch inpolarity of the charges adjacent to the sides 14 and 16 of the web 12contributes to an improvement in the charge density of the charged webas well as an improvement in the lifetime of the charge in the web. Thespecific reasons for this observed improvement are currently unknown tothe inventors.

The charging seen when using an apparatus as shown in FIG. 1 is incontrast to the charging of a web according to the prior art asillustrated by U.S. Pat. No. 4,592,815 to Nakao. The apparatus shown inU.S. Pat. No. 4,592,815 charges a web by placing it in contact with adrum and beneath a series of charging bars arranged in a circularpattern about the drum. The bars are charged and the drum is maintainedat a ground. As is shown in FIG. 2, the electrical fields 50 from thecharging bars 52 are driven together near the surface 54 of the web 56and the drum 58. Thus, the fields 50 are very concentrated near thesurface 54 of the web 56. Even at low electric field levels, theconcentration of electric field 50 near the surface 54 of the web 56 maybe sufficient to cause arcing from the charging bars 52 to the drum 58.Thus, it is often difficult to produce a web by the method of U.S. Pat.No. 4,592,815 without grounding the drum or cylinder.

In contrast, the present method, as illustrated in the apparatus of FIG.1, produces webs without the need to ground the drum. As is shown inFIG. 3, the electric field 60 between the single charging bar 62 and thedrum 64 is more spread out across the surface 66 of the web 68. Themetal or otherwise conductive drum or roller 64 may be either grounded(as in U.S. Pat. No. 4,592,815) or biased. A bias on the drum 64 is usedto control the polarity and charge density on both sides of the web 68.Hence, the filtration efficiency and charge retention of the web areimproved.

Another embodiment is shown in FIG. 4 which shows an alternate apparatus110 for cold charging a web in accordance with the present invention.The depicted apparatus 110 generally comprises an uncharged moving web112 having a first side 114 and a second side 116. The web 112 may havea triboelectric charge associated with the relative inherentelectronegative/electropositive nature of the polymer. In addition, theweb 112 may be a web that has been precharged by either a hot or coldcharging process. The uncharged web 112 passes into the apparatus 110with the first side 114 in contact with a first positioning roller 118.The web 112 then passes over a second positioning roller 120 and ontofirst insulating positioning rollers 122 which position the web 112between the first charging wire 124 and the first charging shell 126.The rollers 122 are generally made of insulating materials such as wood,plastic, or ceramic.

In the following discussion, the first charging wire 124 is assumed tobe "negatively charged" (i.e., at a negative potential) and the firstcharging shell 126 is assumed to be positively charged (i,e., at apositive potential). These assumptions are for the ease of describingthe operation of the invention only and are not intended to limit thescope of the invention to those specific relative potentials.

The charges on the first charging wire 124 and the first charging shell126 induce a charge in the web 112 such that there is a relativepositive charge adjacent the second side 116 of the web 112 and arelative negative charge adjacent the first side 114 of the web 112.

The web 112 then passes on to third, fourth, fifth, and sixthpositioning rollers 128-134 before passing onto second insulatingpositioning rollers 136 which position the web 112 between the secondcharging wire 138 and the second charging shell 140. Again, the rollers136 are generally made of insulating materials such as wood, plastic, orceramic.

The charges (i.e., potentials) on the second charging wire 138 and thesecond charging shell 140 induce a change in the polarity of the chargein the web 112 such that there is a relative negative charge adjacentthe second side 116 of the web 112 and a relative positive chargeadjacent the first side 114 of the web 112. The web 112 then passes overthe positioning rollers 142-148 and out of the apparatus 110.

As in the apparatus 10 described in FIG. 1, the apparatus 110 of FIG. 4provides a web which has had the polarity of its charge changed duringthe charging process. Again, the resulting web retains a long lastinghigh charge density. This is shown in the increase and the relativestability of the filtration efficiency of webs treated according to thepresent invention.

Again, there is a general decrease in the number and size of pinholes(due to arcing between the drum and the charging bar) in the chargedweb. As in the apparatus shown in FIG. 1, the apparatus shown in FIG. 4has a dispersed electric field. As is shown in FIG. 5, the electricfield lines 150 from the charging bar 152 to the shell 154 are spreadacross a rather wide area.

FIG. 6 shows yet another alternative apparatus 210 for cold charging aweb in accordance with the present invention. The apparatus 210 isconfigured to charge webs using charging techniques which can vary, asdesired. This can include the charging of webs with a biased metal orotherwise conductive drum or roller together with a biased or groundedcharging bar, similar to the charging technique discussed in conjunctionwith FIG. 1 of the drawings. This can also include the charging of webswith a charging bar and a charging shell substantially surrounding andapart from the charging bar, similar to the charging technique discussedin conjunction with FIG. 4 of the drawings. This can even includedesired combinations of these two charging techniques. Essentially, thisis accomplished by activating or deactivating (or connecting ordisconnecting) desired sections of the apparatus 210 to achieve thecharging technique which is desired.

The apparatus 210 operates upon an uncharged moving web 212 having afirst side 214 and a second side 216. The uncharged web 212 enters theapparatus 210, coming into contact with a first positioning roller 218,which may be either an insulative or conductive roller. The web 212 isthen passed to a series of insulating positioning rollers 220 whichposition the web 212 between a charging wire 222 and a first chargingshell 224. The web 212 is then passed over a positioning roller 226, andis caused to contact a first charging drum 228, which must beconductive. A charging bar 230 is operatively combined with the chargingdrum 228.

Following a transition, at 232, the web 212 is caused to contact asecond charging drum 234. A charging bar 236 is operatively combinedwith the charging drum 234, which is conductive. The web 212 is thenpassed over a positioning roller 238, and is in turn directed to aseries of positioning insulating rollers 240 which position the web 212between a charging wire 242 and a second charging shell 244. Followingthis, the web 212 is passed over a positioning roller 246, which may beeither conductive or nonconductive, and is directed from the apparatus210, at 248.

It will be appreciated that the apparatus 210 essentially constitutes anoperative combination of an apparatus in accordance with FIG. 1 of thedrawings (herein referred to as "Technique I"), and an apparatus inaccordance with FIG. 4 of the drawings (herein referred to as "TechniqueII"). Through selective use of the four sections represented by thecharging wire 222 and the charging shell 224, the charging bar 230 andthe charging drum 228, the charging bar 236 and the charging drum 234,and the charging wire 242 and the charging shell 244, variouscombinations of the techniques previously described for cold charging aweb of material may be developed. This allows the charging technique tobe tailored to the type of web (e.g., material, thickness, etc.) whichis to be operated upon.

For example, it has been found that relatively thin webs can be moreefficiently charged with an apparatus according to FIG. 1 (Technique I).This is achievable by activating the charging bars 230, 236 and theconductive charging drums 228, 234. It has been found that relativelythick webs can be more efficiently charged with an apparatus accordingto FIG. 4 (Technique II). This is achievable by activating the chargingwires 222, 242 and the charging shells 224, 244. It has even been foundthat combinations of the techniques represented by FIG. 1 and FIG. 4 ofthe drawings (herein referred to as "Technique III") are useful incertain circumstances. For example, it has been found that thinpolypropylene webs (on the order of 20 g/m² or less) will show aparticularly enhanced filtration efficiency (on the order of 91.69%) ifcharged with a combination of the techniques represented by FIG. 1 andFIG. 4 (Technique III). Referring again to FIG. 6, this is achievable byactivating the charging wires 222, 242 and the charging bars 230, 236,as well as the charging shells 224, 244 and the charging drums 228, 234,to achieve a charging of the web 212 through combination of thetechniques previously described. Alternatively, for certain webs, it hasbeen found to be advantageous to use an apparatus including only onecomponent of an apparatus according to FIG. 1 (Technique I) and only onecomponent of an apparatus according to FIG. 4 (Technique II). Againreferring to FIG. 6, and as an example, such an apparatus could includea combination of the charging bar 222 and the charging shell 224, andthe charging wire 230 and the charging drum 228. For further flexibilityin charging of the web 212, the apparatus 210 can include additionalcharging wires 230a, 236a associated with corresponding charging drums(in place of the rollers 226, 238), if desired.

The ability to vary charging technique has revealed yet otherimprovements in implementation. For example, it has been found that whenlaminated (multilayer) webs are charged with the technique representedby FIG. 1 (Technique I), and the technique represented by FIG. 4(Technique II), different characteristics result in terms of thefiltration efficiency of each of the layers associated with the treatedlaminated web. Table I illustrates such variation for webs having threelayers, five layers and eight layers, respectively.

Definitions: In order to better understand the terms used herein,including the Tables and Examples to be set forth below, the followingdefinitions consistent with the accepted technical definitions in theindustry, are submitted.

Filtration Efficiency--is the measure of the ability of a web to removeparticles from a flow of (gaseous or liquid) fluid. The filtrationefficiency is related to the penetration ability, P, of small particlesand is calculated as:

    filtration efficiency=(100-P)%.

Filter Quality--is another measure of the ability of a web to removeparticles from a flow of (gaseous or liquid) fluid. The filter quality,q_(F), is related to the particle penetration through a web, P and tothe pressure drop across the web, Δp, according to the formula:

    q.sub.F =[ln(1/P)]/[Δp].

See William C. Hinds, "Aerosol Technology: Properties, Behavior, andMeasurement of Airborne Particles," John Wiley & Sons, New York, p. 170(1982).

                                      TABLE I                                     __________________________________________________________________________           No.      I.sup.a II.sup.b III.sup.c                                                                             IV.sup.d                                    Number of Layers                                                                       3   3   5   5    5   5   8    8                                      Charging Tech.                                                                         I   II  I   II   I   II  I    II                              __________________________________________________________________________            Filt. Eff.                                                                             98.6                                                                              99.92                                                                             98.60                                                                              99.957                                                                            94.70                                                                             94.64                                                                              99.022                                                                             99.9771                       Overall                                                                              Δp 6.9 6.9 7.2 7.4  2.4 2.3 7.7  7.4                                     Filt. Eff.                                                                             88.2                                                                              88.9                                                                              73.5                                                                              80.4                                                                               53.35                                                                             43.5                                                                              51.1                                                                               59.4                           1st Layer                                                                            Δp 2.4 2.2 1.4 1.6  0.6 0.6 1.1  1.1                                     Filt. Eff.                                                                             60.6                                                                              90.8                                                                              56.9                                                                              80.2                                                                               48.2                                                                              50.95                                                                             49.1                                                                               65.8                           2nd Layer                                                                            Δp 2.3 2.3 1.6 1.5  0.6 0.6 1.1  1.1                                     Filt. Eff.                                                                             82.3                                                                              88.5                                                                              47.1                                                                              85.2                                                                               49.75                                                                             50.7                                                                              33.6                                                                               74.8                           3rd Layer                                                                            Δp 2.3 2.1 1.5 1.7  0.6 0.5 1.1  1.2                                     Filt. Eff.       54.5                                                                              82.9                                                                               45.9                                                                              51.4                                                                              32.8                                                                               74.6                           4th Layer                                                                            Δp         1.6 1.6  0.5 0.6 1.1  1.2                                     Filt. Eff.       67.2                                                                              83.3     46.5                                                                              33.5                                                                               71.9                           5th Layer                                                                            Δp         1.5 1.7      0.6 1.1  1.1                                     Filt. Eff.                        34.3                                                                               71.3                           6th Layer                                                                            Δp                          1.1  1.2                                     Filt. Eff.                        58.0                                                                               70.3                           7th Layer                                                                            Δp                          1.1  1.1                                     Filt. Eff.                        54.3                                                                               73.5                           8th Layer                                                                            Δp                          1.1  1.2                             __________________________________________________________________________     .sup.a 0.9 oz/yd.sup.2 layers, thickness of each layer is 214 μm (for      single layer, typical filt. eff. = 53.9, typical Δp = 2.4).             .sup.b 0.9 oz/yd.sup.2 layers, thickness of each layer is 281 μm (for      single layer, typical filt. eff. = 32.6, typical Δp = 1.5).             .sup.c 0.9 oz/yd.sup.2 layers, thickness of each layer is 550 μm (for      single layer, typical filt. eff. = 14.5, typical Δp = 0.55).            .sup.d 0.39 oz/yd.sup.2 layers, thickness of each layer is 205 μm (for     a single layer, typical filt. eff. = 27.5, typical Δp = 0.95).     

Table I shows that laminated webs treated according to FIG. 1 of thedrawings (Technique I) exhibit higher filtration efficiencies foroutside layers than for inside layers, while the opposite result isobserved for laminated webs treated according to FIG. 4 of the drawings(Technique II).

In order to provide a further understanding of the present invention,the following examples primarily illustrate certain more specificdetails thereof.

Equipment:

Filtration Measurement: A TSI Model 8110 automated filter tester wasused for the measurement of media filtration efficiency. Two percentsodium chloride solution (20 g NaCl in 1 liter of water) was aerosolizedby an aerosol generator. The NaCl/water drops in aerosol were heated andNaCl crystallites with a 0.1 μm diameter were formed. The massconcentration of NaCl in the air was 101 mg/m³. Photometry was used todetect the volume concentration of the air in the upstream volume of themedia (C_(u)) and the volume concentration of the air in the downstreamvolume of the media (C_(d)). The penetration ability of the NaClparticles was calculated as:

    penetration=P=[C.sub.d /C.sub.u ](100%),

and filtration efficiency was calculated as:

    filtration efficiency=(100-P)%.

Surface Charge Potential: A Monroe Model 244 Isoprobe ElectrostaticVoltmeter with a 1017E Probe (0.07 in. opening) was connected to aVelmex system which allows webs with dimensions up to 20×38 inches to bescanned with the probe in both the machine (MD) and cross-machine (CD)directions. The measurement system was interfaced with an IBM ATcomputer using DT 2801 I/O system (Data Translation Inc., Marlborough,Mass.). The average and standard deviation of scanned data weresimultaneously computed along with the absolute average (the average ofthe absolute value of the surface charge potential).

EXAMPLE I

Cold Charging using Contact Cloth: Table II lists the conditions andcharging results for a variety of webs charged according to the methoddisclosed in U.S. Pat. No. 4,375,718 to Wadsworth, et al., which isincorporated herein by reference. Table III lists the measured surfacecharge potential for those webs.

                  TABLE II                                                        ______________________________________                                        No..sup.a                                                                          Filt. Eff..sup.b                                                                        Charging Voltage.sup.c                                                                     Filt. Eff..sup.d                                                                      Δp                                                                           q.sub.F                              ______________________________________                                        1    11.2      -20, +20     63.30   0.9  1.176                                2    45.7      -20, +20     98.708  2.2  1.977                                3    45.83     -20, +20     98.426  1.9  2.386                                4    43.5      -20, +20     99.000  2.1  2.193                                5    24.55     -20, +20     84.775  0.9  2.091                                6    41.55     -18.5, +20   98.460  2.45 1.703                                7    71.65     -21, +21     99.937  6.8  1.084                                ______________________________________                                         .sup.a 1 is 1 oz/yd.sup.2 LLDPE; 2 is 1 oz/yd.sup.2 PP with 10% PP grafte     with 6% acrylic acid; 3 is 2 oz/yd.sup.2 recycled PET; 4 is 1 oz/yd.sup.2     PCT; 5 is 1 oz/yd.sup.2 recycled PET; 6 is 1 oz/yd.sup.2 PP; 7 is 3           oz/yd.sup.2 PP.                                                               .sup.b Before charging.                                                       .sup.c kV for top, bottom.                                                    .sup.d After charging.                                                        .sup.e mm of H.sub.2 O.                                                  

                  TABLE III                                                       ______________________________________                                        Surface Charge Potential                                                      Screen Side         Face Side                                                 No..sup.a                                                                            Arith. Avg.                                                                              Abs. Avg. Arith. Avg.                                                                            Abs. Avg.                                ______________________________________                                        1      +206       323       -345     360                                      2       +26       318        -99     175                                      3       -50       456        +70     292                                      4      -207       609       +1031    1031                                     5      +137       263       +231     425                                      6      +327       355       -153     238                                      7      +468       1081      -790     878                                      ______________________________________                                         .sup.a Same samples as in Table II.                                      

Table II shows how the filtering efficiency of various webs can beimproved by cold charging according to U.S. Pat. No. 4,375,718. As isshown in Table III, the charging of the webs by the method mostlyproduces bipolar (i.e., different charges on opposite sides) webs.

EXAMPLE II

Cold Charging using the Apparatus of FIG. 1: Table IV lists theconditions and charging results for a variety of webs charged on anapparatus according to FIG. 1 (Technique I). Table V lists the measuredsurface charge potential for those webs.

                  TABLE IV                                                        ______________________________________                                        No..sup.a                                                                          Filt. Eff..sup.b                                                                        Charging Voltage.sup.c                                                                     Filt. Eff..sup.d                                                                      Δp                                                                           q.sub.F                              ______________________________________                                         8   71.65     +20 (charged 2×)                                                                     99.317   6.83                                                                              0.730                                 9   71.65     +20 (charged 1×)                                                                     98.610  6.8  0.629                                10   41.55     +22 (charged 1×)                                                                     98.308  2.4  1.700                                11   41.55     +21 (charged 2×)                                                                     98.718  2.5  1.743                                12   76.45     +20 (charged 2×)                                                                     98.063  9.7  0.407                                13   24.55     +22 (charged 2×)                                                                     89.010  1.1  2.007                                14   24.55     +22 (charged 1×)                                                                     90.33   1.0  2.336                                ______________________________________                                         .sup.a 8 and 9 are 3 oz/yd.sup.2 PP; 10 and 11 are 1 oz/yd.sup.2 PP; 12 i     a meltblown/cotton/meltblown thermally bonded composite; 13 and 14 are 1      oz/yd.sup.2 recycled PET.                                                     .sup.b Before charging.                                                       .sup.c Potential on the wire in kV; the rollers were grounded.                .sup.d After charging.                                                   

It is important to note that additional charging operations, performedin series, may be advantageous in some cases (i.e., charged 3×, 4×, 5×,6×, etc.). This may be of particular interest where the webs are beingprocessed at comparatively higher speeds (e.g., at speeds of 1,000feet/minute, or more).

                  TABLE V                                                         ______________________________________                                        Surface Charge Potential                                                      Screen Side         Face Side                                                 No..sup.a                                                                            Arith. Avg.                                                                              Abs. Avg. Arith. Avg.                                                                            Abs. Avg.                                ______________________________________                                         8      +54       404       -465     481                                       9.sup.b                                                                             +523       576       -637     637                                      10.sup.b                                                                             +570       570       -670     670                                      11     +174       239       -192     212                                      12      -9         31       +185     185                                      13     -143       256       -178     206                                      14.sup.b                                                                              34        201       -179     208                                      ______________________________________                                         .sup.a Same samples as in Table IV.                                           .sup.b Single charge with screen side adjacent the roller at the negative     potential.                                                               

Table IV shows that the process of charging a web with the apparatus ofFIG. 1 (Technique I) produces webs with good filtering efficiencies andexcellent filtration qualities. In addition, Table V shows that theprocess produces bipolar webs with good charge differentials between thetwo sides of the web.

EXAMPLE III

Cold Charging using the Apparatus of FIG. 4: Table VI lists theconditions and charging results for a variety of webs charged on anapparatus according to FIG. 4 (Technique II). Table VII lists themeasured surface charge potential for those webs.

                  TABLE VI                                                        ______________________________________                                        No..sup.a                                                                          Filt. Eff..sup.b                                                                        Charging Voltage.sup.c                                                                     Filt. Eff..sup.d                                                                      Δp                                                                           q.sub.F                              ______________________________________                                        15   71.65     +23, -19 (2×)                                                                        99.956  6.9  1.120                                16   71.65     +21, -19 (1×)                                                                        99.950  6.55 1.160                                17   41.55     +25, -19 (2×)                                                                        96.278  2.3  1.431                                18   41.55     +25, -19 (1×)                                                                        95.525  2.55 1.218                                19   24.55     +25, -19 (2×)                                                                        80.35   0.9  1.808                                20   24.55     +25, -15 (1×)                                                                        81.90   1.0  1.709                                ______________________________________                                         .sup.a 15 and 16 are 3 oz/yd.sup.2 PP; 17 and 18 are 1 oz/yd.sup.2 PP; 19     and 20 are 1 oz/yd.sup.2 recycled PET.                                        .sup.b Before charging.                                                       .sup.c kV for wire, shell (number of chargings).                              .sup.d After charging.                                                   

                  TABLE VII                                                       ______________________________________                                        Surface Charge Potential                                                      Screen Side             Face Side                                             No..sup.a                                                                            Arith. Avg.                                                                             Abs. Avg.  Arith. Avg.                                                                           Abs. Avg.                                 ______________________________________                                        15     +1062     1099       -2208   2208                                      16.sup.b                                                                             +1235     1239       -1678   1678                                      17     +183      297        -30     166                                       18.sup.b                                                                             -37       183        -275    294                                       19     -179      244        -66     164                                       20.sup.b                                                                             -233      283        -126    186                                       ______________________________________                                         .sup.a Same samples as in Table IV.                                           .sup.b Single charge with screen side adjacent the shell at the negative      potential.                                                               

Table VI shows that the process of charging a web with the apparatus ofFIG. 4 (Technique II) produces webs with good filtering efficiencies andexcellent filtration qualities for high basis weight webs (i.e., greaterthan about 1.5 oz/yd²) and produces webs with adequate filtrationefficiencies and filtration qualities for lower basis weight webs (i.e,less than about 1.5 oz/yd²). In addition, Table VII shows that theprocess produces bipolar webs with good charge differentials between thetwo sides of the web. While the "cross-over figure" of 1.5 oz/yd²appears to be particularly relevant to meltblown polypropylene webs,this figure may differ from material to material, and can be readilydetermined by testing of the web involved.

EXAMPLE IV

Accelerated Aging Teats: Samples of the various webs were subjected to atemperature of 137° C. for 10 minutes to simulate the effects of agingon the charge of the web. The results are shown in Table VIII.

                  TABLE VIII                                                      ______________________________________                                        No.   Filt. Eff..sup.a                                                                        Filt. Eff..sup.b                                                                       Δp.sup.a                                                                      Δp.sup.b                                                                      q.sub.F.sup.a                                                                       q.sub.F.sup.b                      ______________________________________                                        2     98.708    92.390   2.2   2.1   1.977 1.227                              6     98.460    97.370   2.45  2.2   1.703 1.653                              7     99.937    99.866   6.8   6.1   1.084 1.084                              8     99.317    99.279   6.83  6.1   0.730 0.809                              9     98.610    98.588   6.8   6.2   0.629 0.687                              10    98.308    97.583   2.4   2.2   1.700 1.692                              11    98.718    97.178   2.5   2.2   1.743 1.622                              12    98.063    96.143   9.7   9.8   0.407 0.332                              15    99.956    99.925   6.9   6.3   1.120 1.142                              16    99.950    99.886   6.55  6.0   1.160 1.129                              17    96.278    95.858   2.3   2.2   1.431 1.447                              18    95.525    94.913   2.55  2.2   1.218 1.354                              ______________________________________                                         .sup.a Before accelerated aging.                                              .sup.b After accelerated aging.                                          

The webs produced according to the methods of the present invention showexcellent response to the accelerated aging test. The filteringefficiency and the filtration quality are maintained at consistentlevels, at least as good as the prior art.

EXAMPLE V

Biased Charging Drum Charging: Low basis weight (1 oz/yd²) and highbasis weight (3 oz/yd²) polypropylene webs were charged with anapparatus according to FIG. 1 (Technique I). In all cases, the chargingbar was biased to apply a positive charge while the charging drum wasbiased to apply a negative charge according to Table IX. The height ofthe charging bar above the conductive charging drum was in each case 1.0inches. Table X shows conditions and charging results for the webscharged on an apparatus according to FIG. 1 (Technique I), and Table XIlists the measured surface charge potential for those webs.

                  TABLE IX                                                        ______________________________________                                                 Charging Voltages (kVDC)                                             No..sup.a  Bar 1  Bar 2      Drum 1                                                                              Drum 2                                     ______________________________________                                        21         +10.5  +10.5      -10.5 -10.5                                      22         +16    +16        -4    -4                                         23         +15    +15        -4    -4                                         24         +9     +9         -9    -9                                         ______________________________________                                         .sup.a 21 and 22 are 3 oz/yd.sup.2 polypropylene; 23 and 24 are 1             oz/yd.sup.2 polypropylene.                                               

                  TABLE X                                                         ______________________________________                                        No..sup.a                                                                              Filt. Eff..sup.b                                                                       Filt. Eff..sup.c                                                                           Δp                                                                           q.sub.F                                   ______________________________________                                        21       71.65    97.308       6.45 0.560                                     22       71.65    98.607       5.2  0.822                                     23       41.55    99.191       2.5  1.927                                     24       41.55    98.844       2.5  1.984                                     ______________________________________                                         .sup.a Same as in Table IX.                                                   .sup.b Before charging.                                                       .sup.c After charging.                                                   

                  TABLE XI                                                        ______________________________________                                        Surface Charge Potential                                                      Screen Side             Face Side                                             No..sup.a                                                                            Arith. Avg.                                                                             Abs. Avg.  Arith. Avg.                                                                           Abs. Avg.                                 ______________________________________                                        21     +100      356        -238    284                                       22     -76       287        -315    351                                       23     +540      540        -592    592                                       24     +527      527        -505    505                                       ______________________________________                                         .sup.a Same as in Table IX.                                              

As is seen in Tables IX-XI, webs are produced using biased chargingdrums. Contrary to the expectations of the prior art, the webs exhibithigh values of filtering efficiency and filtration quality.

EXAMPLE VI

Charging of Anionically Grafted Polyolefin: A polyolefin web wasprepared by mixing an original polypropylene with a polypropylene thathad been grafted with 6 wt. % acrylic acid. The polyolefin mixture wascharged by a variety of methods. Table XII shows conditions and chargingresults for the webs.

                  TABLE XII                                                       ______________________________________                                        No..sup.a                                                                           Chg. Method.sup.b                                                                        Filt. Eff..sup.c                                                                        Filt. Eff..sup.d                                                                      Δp                                                                            q.sub.F                              ______________________________________                                        25    A          45.70     98.708  2.2   1.977                                26    B          38.25     99.103  2.25  2.095                                27    B          45.70     98.695  2.05  2.117                                28    C          45.70     97.33   2.6   1.39                                 29    C          45.70     96.37   2.5   1.33                                 ______________________________________                                         .sup.a 25, 27, 28, and 29 are 10 wt. % grafted polypropylene in 90 wt. %      ungrafted polypropylene; 26 is 5 wt. % grafted polypropylene in 90 wt. %      ungrafted polypropylene.                                                      .sup.b A is the contact cloth method of U.S. Pat. No. 4,375,718 to            Wadsworth, et al.; B is the method using the apparatus of FIG. 1; C is th     method using the apparatus of FIG. 4.                                         .sup.c Before charging.                                                       .sup.d After charging.                                                   

Table XII shows that the web is efficiently charged to acceptable levelsso as to produce a web with substantially good filtering efficiency andfiltration quality. Upon accelerated aging testing, sample numbers 25and 28 also showed good retention of the charge. Sample number 25 had afiltering efficiency of 92.390 and Sample number 28 had a filteringefficiency of 87.64. As was discussed above, these results areunexpected in light of the nature of the mixture. The presence of anionsgrafted to the polyolefin would lead a person having skill in the art topredict that the charge would bleed off the web with time.

EXAMPLE VII

Charging at Elevated Temperature: Table XIII lists the conditions andcharging results for a variety of webs charged on an apparatus accordingto FIG. 1 (Technique I) and an apparatus according to FIG. 4 (TechniqueII), at ambient and elevated (150° F.) temperatures. Such charging wasperformed on both polypropylene and polyester webs.

                                      TABLE XIII                                  __________________________________________________________________________                               Charge Potential (V)                                                          Outside  Inside    NaCl                                         Fabric Temp..sup.d                                                                    Control                                                                             Arith.                                                                            Absol.                                                                             Arith.                                                                             Absol.                                                                             Filt.                                                                              Δp                   Sample.sup.a                                                                         Technique                                                                           When Charged                                                                          Filt. Eff.                                                                          Avg.                                                                              Avg. Avg. Avg. Eff..sup.e                                                                         (mm water)                 __________________________________________________________________________    0.75 oz. PP                                                                          I.sup.b                                                                             150° F.                                                                        28.43 143 21.7 -437 437  94.8 1.7                               I     Ambient 28.43 154 210  -422 422  94.3 1.7                               II.sup.c                                                                            150° F.                                                                        27.9  83  177  -78  282  90.8 1.6                               II    Ambient 27.9  169 170  -130 220  92.7 1.65                       3 oz. PP                                                                             I     150° F.                                                                        68.2  162 413  -624 659  95.99                                                                              6.2                               I     Ambient 68.2  154 355  -1103                                                                              1120 98.7 6.3                               II    150° F.                                                                        68.2  625 898  197  482  99.593                                                                             6.2                               II    Ambient 68.2  565 873  -1352                                                                              1367 99.859                                                                             6.4                        1 oz. PET                                                                            I     150° F.                                                                        11.6  6   48   -105 116  32.7 0.5                               I     Ambient 11.6  -128                                                                              233  -520 526  52.5 0.5                               II    150° F.                                                                        11.6  -225                                                                              229  -278 297  45.2 0.6                               II    Ambient 11.6  280 295  -132 161  52.5 0.6                        __________________________________________________________________________     .sup.a Melt blown (MB) webs.                                                  .sup.b Wire height above steel roll of 1 inch, 18-20 kV positive potentia     on wire and bias of 1-2 kV negative potential on both rolls.                  .sup.c Wirein-shell configuration with 5inch diameter aluminum shells, +2     kV potential on the wire, -24 kV on each shell and 5 wooden dowels, 1inch     diameter.                                                                     .sup.d MB webs were laced around the collector cylinder of a 20inch           horizontal MB line and hot air was blown onto the webs which were heated      such that they were all at 150° F. upon entering the charging unit     .sup.e Challenged by aerosol of 0.1 micron sodium chloride particles          travelling at 5 cm/s.                                                    

Table XIII shows that the process of charging a web with either theapparatus of FIG. 1 (Technique I) or the apparatus of FIG. 4 (TechniqueII) produces webs with good filtering efficiencies and excellentfiltration qualities, and that these results are not adversely affectedby elevated temperature.

EXAMPLE VIII

Charging at High Humidity: Table XIV lists the conditions and chargingresults for a variety of webs charged on an apparatus according to FIG.1 (Technique I) and an apparatus according to FIG. 4 (Technique II), attypical (40%) and high (95%) humidities. Such charging was performed onpolypropylene webs of varying weight and fiber diameter.

                                      TABLE XIV                                   __________________________________________________________________________    Samples      Typical.sup.a                                                                            Tech. I.sup.b                                                                            Tech. II.sup.c                                   Fiber  Filt.                                                                             Δp                                                                             Filt.                                                                             Δp                                                                             Filt.                                                                              Δp                              Weight                                                                              Dia. (μm)                                                                         Eff.                                                                              (mm water)                                                                           Eff.                                                                              (mm water)                                                                           Eff. (mm water)                            __________________________________________________________________________     33 g/m.sup.2                                                                       φ = 2.14 μm                                                                   53.9                                                                              2.4    97.3                                                                              2.4    95.0 2.7                                    52 g/m.sup.2                                                                       φ = 2.12 μm                                                                   70.0                                                                              3.6    99.10                                                                             3.8    99.45                                                                              4.1                                   101 g/m.sup.2                                                                       φ = 2.20 μm                                                                   88.9                                                                              6.0    99.22                                                                             7.5    99.999                                                                             7.0                                   13.6 g/m.sup.2                                                                      φ = 2.05 μm                                                                   27.5                                                                              0.95   85.1                                                                              1.2    63.2 1.1                                    33 g/m.sup.2                                                                       φ = 5.50 μm                                                                   14.5                                                                              0.55   60.2                                                                              0.6    59.6 0.8                                   __________________________________________________________________________     .sup.a Typical values at 70° F. and at 40% humidity.                   .sup.b Wire height above steel roll of 1inch, 18-20 kV positive potential     on wire and bias of 1-2 kV negative potential on both rolls; at 95%           humidity and 70° F.                                                    .sup.c Wirein-shell configuration using only one aluminum shell with a        diameter of 5inches and with three 13/8inch diameter wood dowels; at 95%      humidity and 70° F.                                               

Table XIV shows that the process of charging a web with either theapparatus of FIG. 1 (Technique I) or the apparatus of FIG. 4 (TechniqueII) produces webs with good filtering efficiencies and excellentfiltration qualities. For relatively thin webs, Technique I appears tobe especially satifactory and is not adversely affected by elevatedhumidity. For thicker webs, the filtration efficiency is remarkablyincreased using Technique II. This offers greater flexibility dependingon the nature of the web.

EXAMPLE IX

Cold Charging of Laminated Fabrics: Table XV lists the conditions andcharging results for a variety of webs charged on an apparatus accordingto FIG. 1 (Technique I) and an apparatus according to FIG. 4 (TechniqueII). Laminated fabrics (meltblown and spunbond layers) were chargedaccording to the conditions listed, in different orientations (i.e.,meltblown layer facing up and meltblown layer facing down). Thespecified orientation (for the meltblown layer) represents theorientation which the identified layer assumed when entering theapparatus (an appratus according to FIG. 6 suitably connected to assumea configuration according to FIG. 1 or a configuration according to FIG.4) from the right side (as viewed in FIG. 6).

                  TABLE XV                                                        ______________________________________                                        Control       Charged                                                         Wt.  Dist..sup.a                                                                          Eff.   Δp                                                                           Tech.                                                                              MB   V     I    Eff. Δp                    ______________________________________                                        20   5.1    31.0   1.5  I    up   35/-1 2.0  94.5 1.6                         20   8.9    31.0   1.5  II   up    35/-32                                                                             1.0  89.7 1.6                         20   8.9    31.0   1.5  II   down  35/-32                                                                             0.6  90.6 1.6                         20   5.1    31.0   1.5  I    down 40/-1 4.0  89.2 1.7                         20   5.1    40.8   2.5  I    up   35/-1 2.6  97.7 2.9                         20   8.9    40.8   2.5  II   up    35/-32                                                                             0.6  94.5 2.8                         20   8.9    40.8   2.5  II   down  35/-32                                                                             0.6  93.1 2.6                         20   5.1    40.8   2.5  I    down 45/-1 6.0  92.7 2.6                         25   5.1    34.3   1.7  I    up   40/-1 5.0  97.5 1.8                         25   8.9    34.3   1.7  II   up    35/-31                                                                             0.6  94.5 2.1                         25   8.9    34.3   1.7  II   down  35/-32                                                                             0.6  91.5 2.0                         25   5.1    34.3   1.7  I    down 40/-1 5.0  91.1 1.9                         25   5.1    45.5   2.9  I    up   40/-1 5.0  98.1 3.0                         25   8.9    45.5   2.9  II   up    35/-32                                                                             0.6  95.3 2.8                         25   8.9    45.5   2.9  II   down  35/-32                                                                             0.6  94.0 3.0                         25   5.1    45.5   2.9  I    down 40/-1 5.0  94.9 3.0                         30   5.1    37.9   2.0  I    up   40/-1 5.5  95.8 2.2                         30   8.9    37.9   2.0  II   up    35/-32                                                                             0.6  96.7 2.4                         30   8.9    37.9   2.0  II   down  35/-30                                                                             0.5  96.7 2.4                         30   5.1    37.9   2.0  I    down 40/-1 5.5  93.9 2.3                         30   5.1    49.9   3.5  I    up   42/-1 5.5  98.7 3.9                         30   8.9    49.9   3.5  II   up    34/-34                                                                             0.6  98.7 4.0                         30   8.9    54.4   3.4  II   up    33/-33                                                                             0.6  98.7 3.8                         30   5.1    54.4   3.4  I    up   40/-1 5.6  98.5 3.8                         40   8.9    46.6   2.6  II   up    35/-32                                                                             0.6  98.5 3.0                         40   5.1    46.6   2.6  I    up   40/-1 6.0  97.2 3.0                         40   5.1    46.6   2.6  I    down 40/-1 6.0  95.6 3.0                         40   8.9    46.6   2.6  II   down  32/-30                                                                             0.4  98.7 2.7                         80   8.9    66.3   4.7  II   up    30/-28                                                                             0.7  99.50                                                                              5.4                         80   8.9    66.3   4.7  II   down  30/-28                                                                             1.7  99.89                                                                              5.3                         ______________________________________                                         .sup.a Distance (in cm.) between the charging wire or bar and the             corresponding drum or shell.                                             

Table XV shows that when webs are charged with an apparatus according toFIG. 1 (Technique I), the orientation of the meltblown layer (of thelaminated web) will have an effect upon the filtering efficiency whichis achieved. It has been found that when the meltblown layer facesupwardly when traversing the second of the two charging drums, a higherfiltering efficiency is achieved. Without wishing to be bound by suchexplanation, it is believed that this results from a need to ensurecontact between the meltblown layer of the laminated web and the surfaceof the second drum to provide a greater permanence to the charge. Whenwebs are charged with an apparatus according to FIG. 4 (Technique II),the orientation of the meltblown layer relative to the chargingapparatus appears to have no appreciable effect upon the filteringefficiency which is achieved.

Although this specification discloses particular embodiments of theinvention, these examples merely describe illustrations of theinvention. Those skilled in the art may suggest numerous rearrangements,modifications and substitutions of parts of the invention withoutdeparting from the spirit of the invention. For example, it will occurto a skilled practitioner in the art that a similar type of charging maybe effected by alternately varying the polarity of the charging bars andwires and the charging rollers and shells rather than varying the sideof the web that is exposed to them. Also suggested are numerousrearrangements, modifications and substitutions of materials forming thewebs without departing from the spirit of the invention.

In particular, it will occur to a skilled practitioner in the art thatthe configuration of the apparatus used for cold charging of the websmay be varied. For example, the apparatus according to FIG. 1 (TechniqueI) is ordinarily comprised of a wire in conjunction with a steel drum.However, different wire and drum materials can be used provided they areappropriately conductive. It is even possible to replace the wire with aseries of pins, or a charging bar, if desired. In such case, the seriesof pins are radially oriented relative to the drum, and arelongitudinally aligned with one another to form a single row of pinsextending longitudinally along the corresponding drum, serving afunction equivalent to the single wire replaced by the series of pins.

The apparatus according to FIG. 4 (Technique II) is also capable ofvariation. For example, the apparatus according to FIG. 4 (Technique II)is ordinarily comprised of a bar in conjunction with an aluminum shell.As with the apparatus according to FIG. 1 (Technique I), the bar can bereplaced with a series of pins which are radially oriented relative tothe shell and longitudinally aligned with one another to form a singlerow extending longitudinally along the corresponding shell.Alternatively, a plurality of rows of pins may be used, forming a radialpattern extending from the center of the shell to its inner surface.Different bar and shell materials can be used provided they areappropriately conductive. For example, the shell can be formed of avariety of conductive materials including metals, conductive plasticsand ceramics provided with a suitable conductive outer coating (formingthe inner surface of the shell).

Also capable of variation are the rollers used to support the web as itprogresses through the overlying shell (e.g., the rollers 122, 136 inFIG. 4, as well as the rollers 120, 128 and 134, 142 if they areinsulative rollers, as well as the corresponding rollers of theapparatus of FIG. 6). As indicated previously, these rollers can beformed of wood, plastic or a ceramic. Such materials may be used informing the rollers either as unitary structures, or as composites.

FIG. 7a shows a unitary roller 250 which includes a body 252 havingopposing ends 254, 256 of a reduced diameter. The ends 254, 256 areappropriately journalled for rotation within and between the end plates258, 260 which comprise the charging apparatus. Unitary rollers are mostuseful when formed of relatively rigid materials, such as some plastics,ceramics, and wood dowels used in supporting fabrics having a width oftwo meters or less.

For less rigid materials (including some plastics and wood), andreferring to FIG. 7b of the drawings, the rollers are preferablyprovided with a metal shaft 264 running through the center to provideadditional structural integrity to the roller body 266. The use of ametal shaft is further recommended as the length of the rollersincreases beyond one or two meters. In any event, a more stable rollerresults, which in turn minimizes bending or distortion of the rollerwhen placed in service.

FIG. 7c of the drawings shows yet another alternative roller 268 usefulin supporting transported webs. In this embodiment, the unitary (outer)roller body 266 is replaced with a roller body 266' formed of a seriesof segmented sections 270. This has been found to be particularly usefulfor rollers formed of wood, although plastic rollers would also benefitfrom such construction. The segmented sections 270 can, and preferablyinclude male elements 272 and female elements 274 which interact tofacilitate assembly of the segmented sections 270 on the central shaft264. The central (metal) shaft 264 also allows the resulting rollers torotate with less vibration and friction. The segmented sections 270preferably have a length on the order of 6 to 10 inches. The male/femaleelements 272, 274 are useful in minimizing potential effects that thejointed segments may have on the electric field during charging of thewebs. Glues which serve as good insulators may be used to improve thejointed connections, if desired. The segmented sections 270 may also bemachined (e.g., on a lathe) following their assembly to form a moreuniform roller, to minimize any effects of the joints on the webs to betransported through the apparatus.

Table XVI lists the conditions and charging results for a variety ofwebs charged on an apparatus according to FIG. 4 (Technique II) usingvarious different types of rollers including rollers formed of wood,plastics and ceramics, as well as segmented wood rollers.

                  TABLE XVI                                                       ______________________________________                                        Basis                            Filt.                                        Wt..sup.a                                                                            Roller.sup.b,c                                                                           Current Voltage                                                                              Eff.  Δp                               (g/m.sup.2)                                                                          Type       (mA)    (V)    (%)   (mm H.sub.2 O)                         ______________________________________                                        30     nylon      1.0     +40/-40                                                                              95.82 1.45                                   60     nylon      0.2     +36/-36                                                                              96.91 3.4                                    80.sup.d                                                                             nylon      0.6     +35/-33                                                                              99.763                                                                              4.6                                    80.sup.d                                                                             joint wood.sup.e                                                                         0.4     +30/-30                                                                              99.868                                                                              4.7                                    60     joint wood.sup.e                                                                         0.2     +35/-33                                                                              98.48 3.7                                    30     joint wood.sup.e                                                                         0.1     +31/-28                                                                              92.10 1.5                                    30     plexiglass 0.4     +40/-38                                                                              95.97 1.6                                    60     plexiglass 0.2     +35/-35                                                                              98.79 3.5                                    80.sup.d                                                                             plexiglass 0.6     +35/-30                                                                              99.907                                                                              5.3                                    80.sup.d                                                                             wood       0.8     +34/-33                                                                              99.950                                                                              5.7                                    60     wood       0.6     +35/-32                                                                              99.454                                                                              3.3                                    30     wood       0.5     +38/-38                                                                              89.84 1.5                                    30     ceramic    0.4     +30/-18                                                                              94.81 1.6                                    60     ceramic    0.6     +30/-25                                                                              98.78 2.7                                    3.5 oz/yd.sup.2                                                                      ceramic    0.4     +35/-33                                                                              99.807                                                                              3.8                                    3.5 oz/yd.sup.2                                                                      wood*      1.6     +43/-43                                                                              99.971                                                                              5.4                                    60     wood*      0.6     +41/-41                                                                              98.41 2.8                                    30     wood*      0.5     +45/-45                                                                              96.26 1.3                                    80.sup.d                                                                             wood*      1.2     +41/-41                                                                              99.961                                                                              5.2                                    30     control                   35.4  1.4                                    60     control                   59.3  3.0                                    3.5 oz/yd.sup.2                                                                      control                   72.5  4.5                                    80.sup.d                                                                             control                   66.3  4.7                                    ______________________________________                                         .sup.a Basis weight of meltblown polypropylene.                               .sup.b Solid rolls of one material utilized unless otherwise specified.       .sup.c Metal shell of 8inch diameter used with all samples except those       marked with an asterisk, which were used with a metal shell of 10inch         diameter.                                                                     .sup.d Laminated spunbond (SB)/meltblown (MB) web (weight of SB/PP in the     laminate was 20 g/m.sup.2).                                                   .sup.e Metal shaft through wood dowel with straight joints every 7inches.

Table XVI shows that charging a web with the apparatus of FIG. 4(Technique II) produces webs with good filtering efficiencies andexcellent filtration qualities irrespective of the composition of therollers used to support the web as it is transported through theapparatus.

Also capable of variation are the charging shells used in an apparatusfor charging a web according to the previously described Technique II(e.g., the shells 126, 140 of FIG. 4 and the shells 224, 244 of FIG. 6).For example, this can include variation in the diameter of the shell. Inthis regard, shells having a diameter of 5, 7, 8 and 10 inches have beenemployed, and have been found to provide satisfactory results. Any ofthe above-listed roller configurations should work well with either an 8inch diameter shell or a 10 inch diameter shell since, in operation, ithas generally been found that 10 inch diameter shells work very wellwhen compared to 8 inch diameter shells (this, in conjunction with solidwood rollers on a test apparatus).

This can also include variations in the metal shell to facilitateinstallation (so-called "lacing up") of the webs in the chargingapparatus. For example, FIG. 8a shows an apparatus 210' whichsubstantially corresponds to the apparatus 210 of FIG. 6. However, theapparatus 210' of FIG. 8a incorporates shells 278, 280 which arelongitudinally separated into two parts along a seam 282. The resultingshell sections 278', 278" and 280', 280" are preferably equivalent insize and shape, and are connected together by a seamless hinge 284. Thisallows the resulting shell sections to be swung open and away from thecentrally disposed rollers 286, 288 of the apparatus 210', allowing freeaccess for purposes of lacing up the web. Any of a variety of knownseamless hinges can be used for such purposes.

FIG. 8b again shows an apparatus 210" which substantially corresponds tothe apparatus 210 of FIG. 6, and which again incorporates shells 290,292 formed as shell sections 290', 290" and 292', 292". However, in thisembodiment, the shell sections are connected by tracks 294 which permitthe shell sections to slide relative to one another, allowing freeaccess for purposes of lacing up the web. Any of a variety of knowntracks can be used for such purposes.

It is also possible to provide a unitary shell which is slidinglyreceived between the opposing end plates of the apparatus, so thatlacing up of the web can be facilitated by sliding the shell laterallyoutwardly from the apparatus (from one of its ends). However, this ispresently considered less preferred due to the cantilevered length ofthe resulting shell, which can at times be significant.

Similar considerations concerning the lacing up of webs can occur inconjunction with an apparatus for charging a web according to thepreviously described Technique I (e.g., the apparatus of FIG. 1 and thecharging bars 230, 236 and drums 228, 234 of FIG. 6), particularly whenconsidering the relatively fragile wires which may form part of such anapparatus. FIG. 9 shows an apparatus 210'" which substantiallycorresponds to the apparatus 210 of FIG. 6, and which incorporatespivotable fixtures 296, 298 for facilitating lacing up of the webs inthe charging apparatus 210'". In this configuration, the charging wires300 are received by the fixtures 296, 298 and the fixtures are each inturn associated with an arm 302 which is pivotally connected to theframe 304 of the apparatus 210'". Lacing up of the web is facilitated bypivoting the fixtures 296, 298 from their normal position adjacent tothe corresponding drum, to a retracted position which allows free accessto the drum.

It has also been found that any of a variety of power supplies may beused to bias the charging wires/bars and the collecting drums/rollers inaccordance with the present invention. For example, both Glassman andSIMCO power supplies have been used for such purposes while achievingsatisfactory results. Other power supplies may be selected and used ifdesired. Depending upon the power supply which is selected for use,potentials reaching 30 kV, 50 kV, and even up to 100 kV or more, can besuccessfully employed in accordance with the present invention.

It has also been found that the charging wire, the charging bar, theouter surface of the drum and/or the inner surface of the shell can becoated with a thin insulating layer of a plastic (e.g., a synthetic), ifdesired. This allows a more intense electrical field to be developed,yet without arcing. Such a film may be on the order of a few mils (e.g.,5-80 mils) in thickness, or other thicknesses appropriate to aparticular configuration.

The temperature range over which a web or film can be processed inaccordance with the present invention is quite broad. However, since itis generally desired to maintain the physical integrity of the web orfilm (including dimension and/or porosity), it is best not to exceed thetemperature at which the material will begin to shrink. Thus, themaximum temperature at or in the vicinity of the charging apparatus willpreferably range from about 250° F. to about 200° F. The temperature ofthe environment in which the foregoing operations may be carried out canvary over a broad range from about 35° F. to about 120° F. (this tendsto be governed more by the physiological tolerances of people in thevicinity of the apparatus to ambient conditions, more so than to anylimitations on the charging process).

It will also occur to a skilled practitioner in the art that any of avariety of webs formed of any of a variety of materials may be subjectedto a similar type of charging as that previously described, includingboth single and multilayered or laminated webs. For example,deep-grooved 4DG fibers (Eastman Chemical Company) have been treatedaccording to the technique represented by FIG. 4 (Technique II). It wasfound that the resulting filtration efficiency increased from 20% to72%. The 4DG fibers have a great deal more surface area which can acceptelectrostatic charging, leading to such improvements. It is alsopossible to use lighter weight meltblown nonwoven webs and/or to makethe meltblown fibers coarser, and to still obtain high filtrationefficiencies and minimal pressure losses making use of the improvementsof the present invention.

It has also been found that the previously described cold chargingtechniques are useful in treating carded webs (pleated or unpleated),especially carded webs adhered to pleated or flat nonwoven webs (indifferent combinations). Examples of such webs are illustrated in FIGS.10A through 10E of the drawings, which were prepared as follows. Thecarded web 306 of FIG. 10A is comprised of a blend of fibers including75% 6.7 dtex polypropylene and 25% 5.5 dtex polyethylene. The carded web306' of FIG. 10B is comprised of a blend of fibers including 75% 6.7dtex polyester and 25% 3.9 dtex polypropylene. Each of these fiberblends were prepared in conventional fiber opening and blendingequipment. Each of the carded webs 306, 306' is combined with a nonwovenweb 308, which serves as a support and which results in the compositewebs 310, 310' of FIGS. 10A and 10B, respectively. In each case, aneedlepunched nonwoven web was used as the nonwoven web 308. However,other nonwoven webs such as spunbonded, thermally bonded, latex bondedand hydroentangled webs may also be used, if desired.

For purposes of testing, as will be discussed more fully below, a sampleof each composite web 310, 310' (FIG. 10A and FIG. 10B) was preparedwithout a lubricant (fiber finish), and with a lubricant (fiber finish)resulting from a gentle washing of the composite samples in warm (110 to140° F.) soapy water, followed by rinsing in warm water (without soap).The washing and rinsing was repeated three times, and the samples werethen air dried (the fibers of the webs without the lubricant wereprepared by repeatedly washing the ordinary fibers in hot water with asurfactant and in pure hot water, followed by air drying, beforecarding).

Such webs were constructed on a production line 312 of the "STRUTO"type, which is schematically shown in FIG. 11 of the drawings. Such aproduction line can incorporate either a vibrating perpendicular lapper314, as shown in FIGS. 11 and 11A, or a rotating perpendicular lapper316, as shown in FIG. 11B. In either case, the resulting products arecharacterized by fibers situated predominantly perpendicular to theplane of such products, and the fibers are molded into wave forms havingan amplitude which is shorter than the length of the fibers.

FIG. 11A shows a vibrating perpendicular lapper 314, which initiallyreceives a web 318 (in this case, a carded web) between a wire grid 320and a guide board 322. The wire grid 320 and the guide board 322 serveto direct the web 318 onto a conveyor belt 324 (which may constitute theconveyor belt of the thru-air oven to be described more fully below). Aforming comb 326 is received by an arm 328, which is in turn driven by abell crank mechanism 330. The gear 330a driving the bell crank mechanism330 meshes with a gear 332a driving a second bell crank mechanism 332,which causes reciprocating movement of a presser bar 334 (whichpreferably incorporates a series of needles). As the web 318 isintroduced onto the conveyor belt 324, the comb 326 and the presser bar334 are alternatingly driven (by the bell crank mechanisms 330, 332)into and out of engagement with the web 318 so that the comb 326produces pleats in the web 318, and so that the presser bar 334compresses the pleats between the wire grid 320 and the conveyor belt324. This results in a pleated web 336, which issues from the vibratingperpendicular lapper 314 as shown.

FIG. 11B shows a rotating perpendicular lapper 316, which similarlyreceives the web 318 for purposes of developing pleats. In thisconfiguration, the web 318 is received between a feeding disk 338 and afixed guide 340. Rotation of the feeding disk 338 causes the web 318 topass from between the feeding disk 338 and the fixed guide 340, and intoengagement with a working disk 342. The working disk 342 has a series ofteeth 344 which form a series of arcuate cavities 346 for receiving theweb 318 as it passes from between the feeding disk 338 and the fixedguide 340. Rotation of the working disk 342 (in a clockwise direction)operates to collect portions of the web 318 within the cavities 346 ofthe working disk 342, and between a wire grid 348 and the conveyor belt324, forming pleats in the web 318. These pleats are then compressed bythe back edges 350 of the teeth 344, producing the pleated web 336.

Further discussion of the production line 312 of FIG. 11 will proceedwith reference to a vibrating perpendicular lapper such as isillustrated in FIG. 11A, although it is to be understood that therotating perpendicular lapper 316 of FIG. 11B is equally useful in suchan apparatus. Referring now to FIG. 11, the fibers forming the unpleatedweb 318 are received from a carding machine 352. As previouslydescribed, the web 318 received from the carding machine 352 (e.g.,travelling at a speed of about 7 m/min) is introduced between the wiregrid 320 and the guide board 322 of the vibrating perpendicular lapper314, and is received upon the conveyor belt 324, which in this caseconstitutes the conveyor belt of a "thru-air" oven 354. The nonwoven web308 (in this case, a needlepunched nonwoven web) is simultaneouslyintroduced (unwound from a feed roller 356) into the vibratingperpendicular lapper 314 so that the nonwoven web 308 comes to rest uponthe conveyor belt 324.

Downward movement of the forming comb 326 of the vibrating perpendicularlapper 314 forms a pleated structure (so-called "micropleats",corresponding to the pleated carded webs 306, 306' of FIGS. 10A and 10B,and the pleated carded webs 336 of FIGS. 11A and 11B) which is caused torest upon a support fabric (corresponding to the nonwoven web 308). Byway of illustration, the carded web 318 can have a weight of about 20g/m², resulting in pleated carded webs 306, 306' having a weight ofabout 110 g/m².

The resulting structure (a combination of the pleated structure and thesupport fabric) then enters the thru-air oven 354, which fuses thethermoplastic fibers of the pleated structure to each other and to thethermoplastic fibers of the support fabric. By way of example, and for athru-air oven 354 having a length of 1 meter, the oven was heated to atemperature of about 135° C., with a conveyor speed (conveyor belt 324)of about 0.2 to 1.0 m/min (with a speed of 1.0 m/min being used forstaple fiber blends of polypropylene/polyethylene and 0.4 m/min beingused for polyester/polypropylene blends). The conveyor speed and theoven temperature will vary with the length of the oven which is used.

If desired, an adhesive web (note the webs 320 of FIGS. 10C and 10D) maybe applied to the top surface of the support fabric (the nonwoven web308) as it is introduced into the vibrating perpendicular lapper 314 aspreviously described (drawn from the roller 364 of FIG. 11), to furtherimprove adhesion of the pleated structure to the support fabric. Thebonded, laminated structure issuing from the thru-air oven 354 is thencollected, either by hand or with a take-up apparatus 358.

Typically, the binder fibers (polyester or polypropylene, asappropriate) will soften and stick to other fibers at a temperaturewhich is 10 to 30° C. lower than the temperature of the base fiber. Anoven 354 having a length of 3.35 meters, an operating temperature offrom 130 to 240° C., and a fabric transport speed of from 1 to 6 m/min,has generally been found to be appropriate. The selected parameters ofoperation will depend upon the softening point of the lowest meltingpoint material in the composite structure. For these reasons, thecomposite webs 310, 310' of FIGS. 10A and 10B each contained 25% of thelower melting point binder fibers mixed with the base staple fibercarded webs (intimate fiber blends resulting from first being mixed in acarding willow and then in two cards with a cross transfer, with scotchfeed between them) to enhance thermal bonding.

The composite web 360 of FIG. 10C was prepared similarly to thecomposite webs 310, 310' of FIGS. 10A and 10B, except that a 0.75 oz/yd²(25 g/m²) meltblown polypropylene web 362 was used instead of aneedlepunched nonwoven web 308. In forming the web 360, an adhesive web320 (comprised of an undrawn low density polyethylene spunbound web witha weight of 50 g/m²) was fed between the pleated carded web 310" and themeltblown web 362 to form an interface between the two, becoming sticky(by partially melting) in the through-air oven 354 and thereby improvingadhesion of the pleated carded web to the meltblown web. Laminatessimilar to those shown in FIGS. 10A and 10B, using meltblownpolypropylene webs with weights of 10 and 20 g/m², respectively, havesimilarly been made with an interposed adhesive web. The use of anadhesive web in conjunction with such laminated webs was found for bestresults to be highly desirable in cases where the pleated staple fiberweb contained 6.7 dtex, 60 mm polypropylene, which did not contain anylower melting point polyethylene binder fibers.

The composite web 366 of FIG. 10D was prepared by depositing a meltblownweb 368 (drawn from the roller 370 of FIG. 11) onto an unpleated cardedweb 372, in a first pass, and by then turning the resulting laminateover and depositing a second unpleated carded web 374 onto the meltblownweb 368, in a second pass. The resulting (three-layer) laminate was thenpassed through the vibrating perpendicular lapper 314 and the thru-airoven 354 in order to produce the large pleats shown (so-called"macropleats"). A scrim netting 376 was also fed onto the conveyor belt324 (drawn from the roller 378 of FIG. 11, in either the same or aseparate pass) and, together with the composite comprised of the webs368, 372, 374, was fed through the thru-air oven 354 to form the largepleats shown and to attach the structure to the scrim 376. An adhesiveweb 320 (e.g., drawn from the roller 364 of FIG. 11) is advantageouslypositioned between the composite comprised of the webs 368, 372, 374 andthe scrim 376 (prior to introduction to the thru-air oven) to improvethe adhesion achieved at this interface. A laminate similar to thecomposite web 366 of FIG. 10D can be prepared with pleated carded webs,in place of the unpleated carded webs 372, 374, if desired, producing acomposite including both micropleats (the pleated carded webs) andmacropleats (the pleated laminate).

The composite web 378 of FIG. 10E was prepared by first applying acarded web 380 to a 0.75 oz/yd² (25 g/m²) meltblown polypropylene web382 (without pleating). The resulting composite was then turned over andpassed through the vibrating perpendicular lapper 314, together with aneedlepunched web 384 (which serves as a support fabric). In this way,the vibrating perpendicular lapper 314 operates to form the relativelylarge pleats shown (macropleats), without putting micropleats into thecarded web 380. The several layers of the resulting composite web 378(including the carded staple fibers, e.g., of polyester withpolypropylene binder fibers, the meltblown web, and the needlepunchedweb) were thermally adhered together upon exiting the thru-air oven 354.Similar laminates have been prepared with an adhesive layer 320 (e.g., alow density polyethylene of 50 g/m²) positioned between theneedlepunched web and the pleated carded web, and with 100%polypropylene, 6.7 dtex, 60 mm fibers without polyethylene binderfibers. In the latter case, very coarse adhesive webs had to be utilizedto assist in thermal bonding of the resulting structures.

To be noted is that the macropleats of FIGS. 10D and 10E can be formedin different ways, other than with a lapper as previously described. Forexample, such pleats may be formed as air laid, wet laid or dry laidpleats using aerodynamic and/or mechanical techniques.

The foregoing webs were then subjected to testing. Table XVII shows theresults of such testing, as follows:

                  TABLE XVII                                                      ______________________________________                                                 Control.sup.a                                                                              Charged.sup.a                                           Sample     Filt. Eff                                                                              Δp  Filt. Eff.                                                                           Δp                                 Description                                                                              (%)      (mmH.sub.2 O)                                                                           (%)    (mmH.sub.2 O)                            ______________________________________                                        FIG. 10A   22.4     .35       79.25  .30                                      (without Lubricant)                                                           FIG. 10A   12.15    .30       76.10  .35                                      (with Lubricant)                                                              FIG. 10B   16.3     .30       29.8   .40                                      (without Lubricant)                                                           FIG. 10B   10.85    .30       25.8   .35                                      (with Lubricant)                                                              FIG. 10C   53.7     3.05      97.92  3.4                                      FIG. 10D   67.65    2.0       99.617 1.3                                      FIG. 10E   69.9     1.7       99.839 1.65                                     "Filtrete"--                  67.2   0.25                                     Electret Filter                                                               (control)                                                                     ______________________________________                                         .sup.a Aerosol of NaCl having a diameter of 0.1 μm was used at a face      velocity of 5.3 cm/s in filtration testing.                              

These results show that the pleated carded polypropylene and polyesterwebs adhered to the needlepunched nonwoven webs (FIGS. 10A and 10B)resulted in rather low pressure drop values of 0.30 to 0.35 mm. This lowpressure drop is attributed to the fact that although these structuresare typically 1.5 to 2.0 cm thick, the carded fibers in the pleated websare predominantly oriented in a vertical plain, perpendicular to thenonwoven substrate. As a result, air can readily flow through channelsdeveloped along the lengths of the fibers.

Nevertheless, the relative bulk and depth of such structures enhancefiltration efficiency. To illustrate this, the foregoing structures wereelectrostatically charged in accordance with the present invention andat room temperature. Filtration efficiency was then measured, yieldingthe results shown in Table XVII.

The filtration efficiency of the polypropylene-based staple fibersamples (FIG. 10A, unfinished and finished fibers, respectively)increased dramatically after being charging on an apparatus according toFIG. 4 (Technique II). This is so even for the finished fibers, and eventhough many fiber finishes absorb moisture and are believed to interferewith the electrostatic charging of such fabrics. It is significant thatthe results achieved with the web of FIG. 10A have a greater filtrationefficiency than conventionally available "Filtrete" (3M) filters, whichare used for electrostatically enhanced filters in home central airsystems. Although the pressure drops exhibited by the samples of thepresent invention are somewhat higher than those exhibited by the"Filtrete" product, such pressure drops are generally in the same rangeof "low pressure drops" which are satisfactory for such applications.

The polyester samples (FIG. 10B, unfinished and finished, respectively)resulted in a somewhat lower level of filtration efficiency than did thepolypropylene samples. Nevertheless, the filtration efficiency of thepolyester samples was notably improved. It is believed that changes inthe chemistry of the polyester fibers, as well as the shape of theresulting web, can improve the results achievable with polyester fibersbeyond those shown in Table XVII. As an example, deep-grooved 4DG fibers(Eastman Chemical Company) have been treated on an apparatus accordingto FIG. 4 (Technique II). It was found that the resulting filtrationefficiency increased from 20% to 72%. The 4DG fibers have a great dealmore surface area which can accept electrostatic charging, leading tosuch improvements.

Testing of the webs of FIGS. 10C, 10D and 10E resulted in extremely highfiltration efficiencies. The resulting filtration efficiencies (chargedstructures) would be satisfactory as HEPA filters in clean rooms, andfor use under other stringent conditions for extremely low aircontamination. The pressure drops associated with such webs are in arange which is satisfactory for such applications. The web of FIG. 10Cexhibited the highest pressure drop, with the webs of FIGS. 10D and 10Eshowing notably lower pressure drops. These pressure drops are largelydue to the greater surface area that the pleated (meltblown) surfaces ofthe webs present (it should be noted here that the carded webs of FIGS.10D and 10E were not pleated before being laminated with the meltblownpolypropylene web).

If desired, the foregoing composites may also be electrostaticallycharged with an apparatus according to FIG. 1 (Technique I), or anapparatus according to FIG. 6 (Technique III).

It is also possible to use lighter weight meltblown nonwoven webs and/orto make the meltblown fibers coarser, and to still obtain highfiltration efficiencies and minimal pressure losses making use of theimprovements of the present invention. Also, in laminates which employan adhesive web (e.g., the composites of FIGS. 10C and 10D), muchlighter weight adhesive webs can be used such as meltblown adhesive webswhich may be applied uniformly in amounts as small as 2-4 g/m².

The excellent filtration characteristics of the folded composites of theinvention together with their compression behavior (compressive rigidityand high resilience to repeated loading) give these composites acombination of unique properties. The composites also can be used forsound and thermal insulation; in combination with their filtrationproperties they also exhibit excellent liquid retention properties.

The above examples and description fully disclose the present inventionincluding preferred embodiments thereof. The invention, however, is notlimited to the precise embodiments described herein but includes allmodifications encompassed with the scope and spirit of the followingclaims.

The appended claims set forth various novel and useful features of theinvention.

What is claimed is:
 1. A method for electrostatically charging acomposite web having at least two layers including a first layer formedof a carded web and folded to define a series of pleats, and a secondlayer formed of a nonwoven web and including a planar surface, whereinthe pleats of the carded webs are attached to the planar surface of thenonwoven web, the method comprising the steps of:generating pluraldispersed non-arcing electric fields between plural electrode means,each having an electrically conductive curved surface, and acorresponding number of single conductive elements spaced from thecurved surface of the corresponding electrode means; passing thecomposite web between the plural electrode means and the correspondingconductive elements, subjecting the composite web to the pluraldispersed non-arcing electric fields between the plural electrode meansand the corresponding conductive elements; exposing a first side of thecomposite web to a first polarity charge upon passage between a firstelectrode means and a first corresponding conductive element, and to asecond, opposite polarity charge upon passage between a second electrodemeans and a second corresponding conductive element; and exposing asecond side of the composite web opposite the first side to the secondpolarity charge upon passage between the first electrode means and thefirst corresponding conductive element, and to the first polarity chargeupon passage between the second electrode means and the secondcorresponding conductive element.
 2. The method of claim 1 wherein thefirst electrode means includes a conductive shell, wherein the curvedsurface is an inner surface of the shell, and which further comprisesthe step of spacing the composite web from the shell and the firstconductive element as the composite web passes through the electricfield between the shell and the first conductive element.
 3. The methodof claim 2 wherein the first conductive element is a charging bar. 4.The method of claim 2 wherein the second electrode means includes aconductive shell having a curved surface, wherein the second conductiveelement includes a charging bar associated with the conductive shell,and which further comprises the step of spacing the composite web fromthe shell and the charging bar as the composite web passes through theelectric field between the shell and the charging bar.
 5. The method ofclaim 4 wherein the second conductive element is a charging bar.
 6. Themethod of claim 1 wherein the first electrode means includes aconductive drum, wherein the curved surface is an outer surface of thedrum, and which further comprises the step of placing the composite webin contact with the outer surface of the drum as the composite webpasses through the electric field between the drum and the firstconductive element.
 7. The method of claim 6 wherein the firstconductive element is a charging wire.
 8. The method of claim 6 whereinthe second electrode means includes a conductive drum, wherein thecurved surface is an outer surface of the drum, and which furthercomprises the step of placing the composite web in contact with theouter surface of the drum as the composite web passes through theelectric field between the drum and the first conductive element.
 9. Themethod of claim 8 wherein the first conductive element further includesa charging wire.
 10. The method of claim 1 wherein the electric field isgenerated between about 1 kVDC/cm and about 16 kVDC/cm.
 11. The methodof claim 10 wherein the electric field is generated between about 4kVDC/cm and about 13 kVDC/cm.
 12. The method of claim 10 wherein theelectric field is generated between about 7 kVDC/cm and about 12kVDC/cm.